1. Field of the Invention
The present invention relates generally to an optical device employing a metal halide lamp as a light source and, more particularly, to an illuminator with a metal halide lamp having an enhanced starting property and also to an image display such as, for example, a projector employing the above illuminator.
2. Description of Related Art
Metal halide lamps, filled with metal halides such as, for example, dysprosium iodide, neodymium iodide, or cesium iodide, rare gas such as, for example, argon gas as auxiliary starting gas, and mercury, generally have superior luminous characteristics, a relatively high luminous efficacy, and a high durability. For this reason, the metal halide lamps are in wide practical use today for indoor and outdoor lighting and in various image displays such as, for example, liquid crystal projection type displays and projectors including an overhead projector (OHP).
Operating the metal halide lamps which make use of luminescence of the encapsulated materials in the arc discharge requires an electric circuit (generally including a starter and a ballast) for starting and stably maintaining the discharge. By way of example, in the case of a 150-W metal halide lamp, the circuit therefor is designed so as to provide a lamp voltage of about 85 volts and a lamp current of about 1.78 amperes after reaching a steady light-up condition.
A higher voltage than the rated lamp voltage is, however, required to initiate the discharge in the metal halide lamp. The reason for this is that metal halides, mercury halides, free halogen gas and like materials in the metal halide lamp tube capture electrons in the discharge space, as can be known from, for example, "Electric Discharge Lamps" (MIT Press 1971, pp254) written by J. F. Waymouth. Accordingly, the circuits for metal halide discharge lamps generally have equipment for generating a high voltage necessary to initiate the discharge. With such equipment, a high voltage of about 10 kilovolts is applied to the lamp for about 2 seconds at a frequency of 100 Hz, for example, only at the starting of the lamp.
The application of such high voltage of about 10 kilovolts to the lamp, even though only at the starting, occasionally causes a problem associated with safety. Unless a sufficient distance is ensured between a power feeder line to the lamp and neighboring components, there arises a problem such as, for example, damage of the components, electric shock, or the like at the time of applying the high voltage. Considering humidity, the power feeder line to the lamp must be sufficiently spaced away from the neighboring components.
Furthermore, it is necessary to make the circuit of the metal halide lamp as small as possible, because the circuit is in most cases accommodated in an instrument or installed at a high position. Conventional circuits for the metal halide lamps could not satisfy both the requirement to reduce the size thereof and the requirement for safety. In order to cope with both the safety factor and the cost factor, it is desired to minimize the voltage required for starting the lamp.
Especially in image displays such as, for example, OHPs or liquid crystal projectors which should be small-sized and portable, it is necessary to reduce the starting voltage of the lamp.